Aliphatic alpha, beta-unsaturated acids have been already obtained from the corresponding saturated acids by a dehydrogenation in the presence of oxygen on solid catalysts; such catalysts, according to the French Pat. No. 2,135,786, are for instance mixed phosphates of iron and bismuth. Still another patent (U.S. Pat. No. 3,948,959) relates to the use of mixed phosphates of iron and alkali or alkaline earth elements.